Impression Creep Technique Research Articles

Effect of Welding Process on Microstructure and Mechanical Properties of Boron Containing Modified 9Cr-1Mo Steel

The influence of Shielded Metal Arc (SMA) welding and Activated Tungsten Inert Gas (A-TIG) welding on the microstructure and mechanical properties of boron containing Modified 9Cr-1Mo steel has been investigated. The tensile strengths of SMA weld joints at room temperature and 550℃ (730, 710 MPa) are higher than those of A-TIG weld joints (520, 465 MPa); whereas A-TIG welds (66, 18J) show better impact energies at room temperature and 0℃ than SMA welds (53, 12J). Creep resistances of both the weld metals, as estimated by impression creep technique are comparable; while, the A-TIG heat affected zone (HAZ) possesses higher creep resistance than the SMA HAZ. The fracture toughness of the A-TIG weld metal is inferior (64 kJ.m-2) to that of SMA weld metal (181 kJ.m-2) and brittle fracture is seen. It has been observed that the multiple passes of SMA process are advantageous in modifying the delta ferrite present in the weld metal although they cause severe damage to the HAZ microstructure.

Estimation of deformation behaviour at different weld zones of FSWed 2014 aluminium alloy through impression creep technique

Impression creep (IC) tests of the best-welded specimen at three dissimilar weld zones of friction stir welded (FSWed) 2014 grade aluminium alloy welded with optimized process parameters were carried out at a temperature of 310, 330 and 350 °C and stress regimes of 21, 24 and 27 MPa. The creep deformation properties of the localized zones viz: nugget zone (NZ), thermo-mechanical affected zone (TMAZ) and heat-affected zone (HAZ) of the FSWed material including the base material (BM) were revealed using the power law creep equation and correlated with the variation in microstructural properties. The steady-state creep rate of the individual zones along with the BM was analyzed. The other creep parameters such as stress exponents (n) and apparent activation energy for the IC deformation procedure (Qic ) were calculated henceforth. From the calculated n and Qic value, the creep deformation process in the BM as well as in TMAZ and HAZ of the welded material was estimated to be through dislocation glide. Whereas the NZ deforms by grain boundary sliding and this may be due to the heterogeneity in the microstructure. The micro-hardness after IC test was increased for each sample. The outcomes are concluded with the microstructural study of the creep tested sample through optical and field emission scanning electron microscope. The particles agglomeration and dislocation structures at NZ were confirmed by transmission electron microscope (TEM). To observe the composition influence of the precipitated second phase particles on creep behaviour, an energy dispersive x-ray study was also carried out.

Effect of Ni on Microstructure and Creep Behavior of A356 Aluminum Alloy

The effect of 0.25, 0.5 and 1 wt% Ni addition on the impression creep behavior of the cast A356 alloy was investigated. Optical and scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) were used for examination of the microstructure. The alloy’s creep properties were investigated using the impression creep technique under normalized stress of 0.022–0.03 (corresponding to 600–675 MPa) and temperature of 473–513 K. The results showed that the creep properties of A356 alloy were improved by the addition of Ni. The improved creep properties were attributed to the modification of eutectic silicon and the formation of Ni-rich intermetallics. Calculating the values of stress exponent (n) and creep activation energy (Q) indicated that the dominant mechanism was the lattice self-diffusion climb controlled and Ni had no effect on the creep mechanism.

The Effects of Zr Addition on the Microstructure and Mechanical Properties of A356–SiC Composites

The influences of Zr on the microstructure of A356–15 vol% SiC composites, such as the distribution of SiC particles in the matrix, the grain size and the hardness and impression creep between 225 and 275 °C, were examined. These composites were prepared using the stir casting technique. The addition of Zr forms Al3Zr phases, which can act as heterogeneous nucleation sites, thus resulting in grain refinement. Furthermore, Zr leads to a more uniform distribution and an increased area fraction of SiC particles. Moreover, the addition of only 0.1 wt% Zr significantly increased the Brinell hardness by approximately 18% when compared to the composite without added Zr. The addition of a small amount of Zr (0.1 wt%) is sufficient to increase the Vickers microhardness by approximately 34% when compared to the composite without added Zr, indicating the effect of solid solution strengthening on the composite matrix. The high-temperature stability test using the impression creep technique demonstrated that the A356–15 vol% SiC composite with 0.1 wt% added Zr shows improved creep resistance. However, the creep resistance of composite samples with higher than 0.1 wt% added Zr decreases due to grain boundary sliding occurring in the A356–15vol% SiC composites with too much added Zr.

Impression creep behavior of extruded ZK60 and ZK60+1 %Y magnesium alloys

Abstract The creep behavior of the extruded Mg alloys ZK60 and ZK60+1 %Y was investigated by means of the impression creep technique. The tests were carried out at a temperature in the range of 175–225 °C and under punching stress in the range of 25–100 MPa. The results showed that a 1 wt.-% yttrium addition could improve the creep resistance of ZK60. The creep mechanism of the extruded Mg alloy ZK60 consists of grain boundary diffusion by grain boundary sliding and that of the extruded Mg alloy ZK60+1 %Y grain boundary slip initiated by Mg. The microstructure of the extruded Mg alloy ZK60 consists mainly of the Mg phase and that of the Mg alloy ZK60+1 %Y in the extruded condition is composed of an α-Mg matrix and β-Mg3Zn3Y2 phase. After the press-in creep, the extruded Mg alloy ZK60 precipitates MgZn2 and MgZn phases and a small amount of the Zr-Zn phase, and the extruded Mg alloy ZK60+1 %Y precipitates MgZn2, MgZn, Y-Zn and the rare earth phase Mg-Y. The precipitation of the rare earth phase is represented by a discontinuous thin strip and a fine granular uniform distribution in the matrix. The precipitation of the rare earth compound phase shows high thermal stability, which can improve the creep resistance of the alloy. The Y-containing alloy ZK60 manifests higher threshold stress and better creep resistance due to the formation of the rare earth thermally stable particle.

Characterizing Creep Behaviour of Modified 9Cr1Mo Steel by using Small Punch Impression Technique for Thermal Powerplants

Modified 9Cr1Mo (P91) having ferritic/martensitic structure is a candidate material for thermal powerplant industry due to its enhanced creep resistance. Fusion welding of P91 steel nurtures heat affected zone (HAZ) which gives rise to infamous type IV failure/cracking. The origination of type IV cracking still remains ambiguous mainly due to sampling constraints from P91 - HAZ subzones owing to thin width of HAZ. In present work, this issue is investigated with the help of small punch impression creep technique. Thermo – mechanical simulator Gleeble®-3800 is used to physically simulate HAZ subzones of P91 steel representing HAZ – CG, HAZ – FG and HAZ – IC. Impression creep tests done on parent metal P91 and its simulated HAZ subzones at a stress level of 270 MPa and 600℃ reveal HAZ – IC as the weakest subzone followed by HAZ – FG. Coarsening of chromium rich carbides on prior austenite grain boundaries (PAGBs) associated with blended phase transformations in HAZ – IC render it as the most degraded HAZ subzone in P91 steel thus becoming an initiating location for type IV cracking. Furthermore, presence of relatively fine carbides combined with less inter-lath spacing in P91 steel and its HAZ – CG contribute towards retarding creep rate leading to an increase in heterogeneous creep behaviour among HAZ subzones and parent metal.

Finite Element Analysis of Impression Creep

Impression creep test which uses flat cylindrical indenter has attracted many researchers to study the creep response of different materials. It is material non-invasive technique and is extensively used to predict the creep properties of different materials. Present work deals with the hypothetical finite element analysis of the impression creep technique in order to establish the relationships for transferability of impression creep data to uniaxial creep. In this investigation, detailed finite element analysis has been carried out considering the effect of specimen thickness, indenter fillet radius and material’s response to creep deformation and their influence on the conversion factors. In the parametric study, the analysis was carried out by varying geometry of the specimen and the test material in order to study their response on the conversion factors. It was found that the conversion factors are sensitive to the relative dimension of the indenter. Also the effect of fillet radius on the flat cylindrical indenter is studied to account for the stress concentration at the indenter corner. The dependence of conversion factors on different variables of impression creep has been discussed in detail.

Impression creep behavior of magnesium alloy ZK60

Abstract Creep behavior of the ZK60 magnesium alloy was investigated using impression creep technique. The tests were carried out under constant punching stress at high temperatures (175, 200 and 225 °C) under low stresses (25, 50, 75 and 100 MPa) and at low temperatures (100, 125 and 150 °C) under high stresses (100, 125, 150 and 175 MPa). The creep mechanism of the extruded ZK60 magnesium alloy is grain boundary sliding by dislocation climbing obtained at low temperature and high pressure. At high temperature and under low pressure, the creep mechanism of the extruded ZK60 magnesium alloy is grain boundary diffusion by grain boundary sliding. The initial microstructure of the extruded ZK60 magnesium alloy was composed of mainly Mg phase and a little MgZn phase. Under the conditions of low temperature and high pressure, MgZn2 phase precipitated during creep deformation. Under the conditions of high temperature and low pressure, MgZn2 phase and a small amount of Zr-Zn phase precipitated during creep deformation. The creep resistance of magnesium alloy ZK60 under the conditions of low temperature and high pressure is better than under the conditions of high temperature and low pressure.

Impression Creep Behaviour of Extruded Mg-Sn Alloy

Mg-Sn alloys contain thermally stable Mg2Sn phase, and are proposed as heat-resistant alloys for automobile engine applications. In this study, the creep behaviour of Mg-5Sn alloy was investigated using impression creep technique. The impression creep tests were carried out under constant punching stress in the range of 80-320 MPa at temperatures 373-573 K, for dwell times up to 5 hours. The results highlight that creep of Mg-5Sn alloy was load and temperature dependent, i.e. increasing the load and temperature resulted in larger creep deformation and hence to higher creep rates. From the creep curves, the stress exponent and the activation energy were estimated and the creep mechanism was identified.

Impression Creep Characteristics of Mg-Sn-Ag Alloy

Magnesium alloys with enhanced thermal stability are being developed for automobile engine applications. The available commercial Mg-alloys are usually alloyed with aluminum that are thermally stable only for T < 150o C. Development of new Mg-alloys is underway and Mg-Sn alloys are a promising option. In Mg-Sn alloy, the Mg2Sn phase has high thermal stability and is expected to enhance the high temperature properties. In this study, Mg-5Sn alloy is incorporated with Ag as a minor alloying element (0.175 wt. %). The creep behavior of the Mg-Sn-Ag alloy is investigated using the impression creep technique. The impression creep tests were carried out under punching stress in the range of 80-320 MPa and temperature of 373-573 K, for dwell times up to 5 hours. The results highlight that creep of the alloy was load and temperature dependent, i.e. increasing the load and temperature resulted in higher creep rates.

Impression creep behaviour of squeeze-cast Ca and Sb added AZ91 magnesium alloy

Impression creep technique has been employed to investigate creep behaviour of the squeeze-cast AZ91 Mg alloy modified by the addition of Ca and/or Sb. All the modified alloys exhibit superior creep resistance than the base AZ91 alloy. The combined additions of Ca and Sb are more effective in improving creep resistance than the individual additions. Individual Ca added AZX911 results superior creep resistance than the Sb added AZY911 owing to the higher thermal stability of the Al2Ca phase in the former alloy compared to that of the Mg3Sb2 phase in the later one. The AZXY9120 alloy pertaining 2.0Ca and 0.3Sb (wt%) exhibits the best creep resistance due to reduced amount of β-Mg17Al12 phase and presence of higher amount of dense network of thermally stable Al2Ca phase at grain boundaries. Post creep microstructural observation confirms the ability of the Al2Ca phase to withstand applied high stresses at elevated temperature without undergoing significant changes in its structure. The values of stress exponents and activation energies are in the range of 4.3–6.5 and 111.9±1.1 to 114.9±3.0kJ/mol, which concludes dislocation climb controlled by pipe diffusion is the dominant creep mechanism for all the alloys in the temperature and stress level employed.

Evaluation of creep deformation behaviour of different microstuctural zones of 316LN SS weld joint using impression creep testing technique

Impression creep technique, which uses flat ended cylindrical indenter, is an attractive material non-invasive technique for evaluating creep properties as it enables probing small volume of material and requires short time for testing. The present investigation discusses the application of impression creep technique for characterising the creep behaviour of different microstructural regions in a high nitrogen grade of 316LN SS weld joint. Impression creep tests were carried out at 923 K on the weld metal, the base metal and the heat-affected zone in 316LN SS weld joint. The impression creep curves exhibited the first two characteristic stages of the creep processes, namely, a primary creep stage and a secondary creep stage. The rate at which the cylindrical flat punch penetrated into the test specimen was controlled by the time dependence of the movement of the material underneath the punch. The weld metal, the base metal and the heat affected zone exhibited distinct creep behaviour. The difference in creep behaviour of each zone was correlated with the difference in microstructure and morphology of the weld joint. A comparison was made between the data obtained from impression creep tests and from independent uniaxial creep tests on the weld metal and the base metal. A good correlation was established between impression creep and uniaxial creep results.

Impression creep behavior of a cast MRI153 magnesium alloy

Creep behavior of a cast MRI153 magnesium alloy was investigated using impression creep technique. The tests were carried out under constant punching stress in the range of 360–600MPa at temperatures between 425 and 490K. Microstructure of the alloy was composed of α(Mg) matrix phase besides Mg17Al12 and Al2Ca intermetallic compounds. Stress exponent of minimum creep rate, n, was found to vary between 6.45 and 7. Calculation of the activation energy showed a slight decrease with increasing stress such that the creep activation energy of 115.2kJ/mol under σimp/G=0.030 decreased to 99.5kJ/mol under σimp/G=0.040. The obtained stress exponent and activation energy data suggested that the pipe diffusion dislocation climb controlled creep as the dominant mechanism during the creep test.

Creep deformation behavior of Sn–Zn solder alloys

Creep properties of three Sn–Zn solder alloys (Sn–9Zn, Sn–20Zn, and Sn–25Zn, wt%) were studied using the impression creep technique. Microstructural characteristics were examined using a scanning electron microscope. The alloys exhibited stress exponents of about 5.0. The activation energy for creep was calculated to be ~50–75 kJ/mol with a mean value of 66.3 kJ/mol. The likely creep mechanism was identified to be the low temperature viscous glide of dislocations.

Impression creep behavior of Al–1.9%Ni–1.6%Mn–1%Mg alloy

In the present study, microstructure and creep behavior of an Al–1.9%Ni–1.6%Mn–1%Mg alloy were studied at temperature ranging from 493 to 513K and under stresses between 420 and 530MPa. The creep test was carried out by impression creep technique in which a flat ended cylindrical indenter was impressed on the specimens. The results showed that microstructure of the alloy is composed of primary α(Al) phase covered by a mantle of α(Al)+Ni3Al intermetallic compound. Mn segregated into AlxMnyNiz or Al6Mn phases distributed inside the matrix phase. It was found that the stress exponent, n, decreases from 5.2 to 3.6 with increasing temperature. Creep activation energies between 115kJ/mol and 151kJ/mol were estimated for the alloy and it decreases with rising stress. According to the stress exponent and creep activation energies, the lattice and pipe diffusion- climb controlled dislocation creep were the dominant creep mechanism.

Investigation on the impression creep properties of a cast Mg–6Al–1Zn magnesium alloy

In the present study, the creep behavior of a cast Mg–6Al–1Zn (AZ61) magnesium alloy was investigated using impression creep technique at the temperature range of 423–495K under shear modulus normalized stress (σimp/G) between 0.02 and 0.0425. Based on the obtained results in the applied stress range, the creep behavior of the alloy was divided into two low and high stress regimes. Stress exponent varied in the range of 4–6 and 11–12 at the low and high stress regimes, respectively. Also, creep activation energy (Q) varied in the range of 113–199kJ/mol and 170–188kJ/mol at the low and high stress regimes, respectively. The activation energy values obtained at the low stress regime were between activation energy for magnesium lattice self diffusion (135kJ/mol) and dislocation pipe diffusion (92kJ/mol). Considering the obtained stress exponents and creep activation energies, it can be stated that the climb controlled dislocation creep and power law breakdown are the dominant creep mechanisms at the low and high stress regimes, respectively.

Impression Creep Behaviour of U−6%Zr Alloy: Role of Microstructure

Abstract Creep behavior of U−6%Zr alloy was studied in the temperature range 550−650 °C using impression creep technique. Creep curves at 575 °C for various stresses are not smooth and exhibited some waviness. Sometimes creep curves show a sudden penetration of the punch which is followed by decrease in impression depth with time. The punching stress versus creep rate data for the above temperature also show a random behaviour. A detailed microstructural investigation was carried out with the help of SEM and EDX to understand this phenomenon. At creep testing temperature, δ-UZr2 phase continuously forms and it takes very long time to complete the reaction. Hence, the reason for waviness may be due to the formation of δ-phase coming out from the supersaturated α-phase.

Application of Impression Creep Technique for Development of Creep Resistant Austenitic Stainless Steel

The present paper highlights the usefulness of impression creep technique as applicable to alloy development studies of a creep resistant austenitic stainless steel which has been used as structural material for Fast Breeder Reactors. The effect of nitrogen on creep deformation behavior of type 316LN stainless steel has been studied using impression creep testing technique. Impression creep tests have been carried out at 923 K on 316LN stainless steel containing 0.07, 0.11, 0.14, and 0.22 wt. % nitrogen, in the punching stress range of 472-760 MPa. The impression creep curves were characterized by a loading strain, a primary creep stage and a secondary creep stage similar to uniaxial creep curves. The tertiary stage observed in uniaxial creep curves was absent. The steady state impression velocity was found to increase with increasing punching stress. It was observed that impression velocity was sensitive to the variation in nitrogen content in the steel. Steady state impression velocity was found to decrease with increasing nitrogen content as was observed in conventional uniaxial creep tests. A good correlation between the impression creep test and the conventional creep test results was observed which demonstrates the applicability of impression creep testing technique for alloy development programme.

Creep studies on U–7%Zr, U–7%Nb and U rich U–Nb–Zr alloys

Creep behavior of U–7%Zr, U–5%Zr–2%Nb, U–3.5%Zr–3.5%Nb, U–2%Zr–5%Nb, U–7%Nb alloys (composition in wt.%) was investigated by the impression creep technique at 630 and 700°C at a stress of 22.2MPa. Creep rate was found to be two orders lower in the binary U–7%Nb than the U–7%Zr alloy. In ternary U–Nb–Zr alloys, the creep rate was found decreasing drastically with the increase in Nb content.

Creep behaviour of U–6%Zr alloy by impression creep technique

In this study, the impression creep behaviour of U–6wt.%Zr alloy was studied in the temperature range 550–650°C at different stresses. The velocity of the punch at different stresses and temperatures was evaluated for the above alloy. The creep curves at 575°C showed a wavy phenomenon at all stresses. The stress exponents and thermal activation parameters of the above alloy were determined. A power law behaviour is displayed in the temperature range 600–650°C with the stress exponents range from 8 to 8.4. The activation enthalpy for the U–6%Zr alloy was found to be independent of stress with an average value of 157kJ/mol.